1. Field of the Invention
The present invention relates to an image forming apparatus that forms a latent image on a photosensitive member by irradiating the photosensitive member with light beams based on image data by causing a plurality of light sources to emit the light beams based on the image data.
2. Description of the Related Art
In an image forming apparatus such as a copying machine and a laser beam printer using a laser beam and employing electrophotography, there has been discussed a technique that traces a plurality of scanning lines simultaneously by using a plurality of laser light sources to attain high speed printing and high resolution (Japanese Patent Application Laid-Open No. 03-200917).
However, due to different light paths of the laser beams, positions at which the laser beams pass in an optical system are varied from each other, and curvatures and inclinations of the laser beams are varied from each other in some cases. In such a case, a pitch between the scanning lines traced by the laser beams is undesirably varied depending on a main scanning position. Therefore, due to the nonuniformity of the pitches of the scanning lines, density non-uniformity occurs in a page, a background, or a predetermined region of a screen image in which uniform density should be kept.
Hereinafter, the above-mentioned issue will be described in detail. FIG. 5 is a diagram illustrating scanning lines when a photosensitive member is irradiated with laser beams emitted from light sources A and B via an optical system. A dashed dotted line indicates an ideal scanning line; a solid line indicates the scanning line by the laser beam from the light source A; and a dotted line indicates the scanning line by the laser beam from the light source B.
The scanning lines by the laser beams from the light sources A and B are curved by different degrees depending on a difference in a passing position or a reflected position in the optical system. As a result, wideness and narrowness are generated in the scanning lines by the laser beams from the light sources A and B.
For example, when the light sources A and B emit laser beams at a main scanning position X1 in the first scanning, irradiated two laser spots are closer to each other than the ideal case (the pitch between the scanning lines is narrow). In contrast, when the light source B emits a laser beam in the first scanning, and the light source A emits a laser beam in the second scanning at a main scanning position X2, the distance of the irradiated two laser spots are larger from each other than the ideal case (the pitch between the scanning lines is wide).
FIG. 6A is a diagram illustrating a latent image potential formed by two laser spots in the narrow state, and FIG. 6B is a diagram illustrating a latent image potential formed by two laser spots in the wide state. A downward direction of a vertical axis indicates a sub-scanning direction; a rightward direction of a horizontal axis indicates a surface potential of a photosensitive member 708; and a leftward direction of the horizontal axis indicates an amount of toner adhered on the photosensitive member 708 by development.
When the photosensitive member 708 is irradiated with laser in a state where a surface thereof is charged by a high negative voltage, a surface potential of the photosensitive member 708 is increased, and a latent image potential is formed. Toner is adhered by development processing on a part in which the latent image potential exceeds a threshold value Vth.
When laser spots of the light sources A and B are close to each other, a combined latent image potential is formed depending on a degree of overlapping of two latent image potentials. When latent images are formed by the laser spots in the narrow state, an overlapping part of latent image potentials is increased as illustrated in FIG. 6A to cause an increase in a region where a combined latent image potential exceeds the threshold value Vth. As a result, a toner adherence width in the sub-scanning direction is widened.
In contrast, when latent images are formed by two laser spots in the wide state, an overlapping part of latent image potentials is reduced as illustrated in FIG. 6B to cause a reduction in a region where a combined latent image potential exceeds the threshold value Vth. As a result, a toner adherence width in the sub-scanning direction is narrowed.
Therefore, density nonuniformity which is like a moiré caused by wideness and narrowness of scanning lines and interference by screen cycle is sometimes generated in a page, a background, or a predetermined region of a screen image in which uniform density should be kept.